This invention relates to the art of controlling the magnetic flux density within magnetic bodies. A varying voltage applied to a conductive winding causes the voltage induced in the winding to have such waveform and magnitude that the integral over time of the induced voltage correlates to desired changes of the magnetic induction level.
Prior art methods, that utilize a conductive winding to control magnetization, generally focus on controlling winding current to control the induction level of a magnetic body. The winding current directly correlates with magnetic field intensity. Changes in magnetic field intensity are coordinated with hysteresis-curve properties of the magnetic material to control the induction level. (As used throughout this disclosure, the following terms are synonymous. xe2x80x9cmagnetization,xe2x80x9d xe2x80x9cmagnetic induction,xe2x80x9d xe2x80x9cflux densityxe2x80x9d and xe2x80x9cinduction levelxe2x80x9d).
Many applications are particularly concerned with how a magnetic body may be demagnetized. Most prior-art demagnetizing methods (that effectively reduce the induction level of a magnetic body to near zero) utilize a declining alternating magnetic field. The alternating magnetic field is usually produced by a conductive winding conducting a declining alternating current. Many different embodiments of this demagnetizing method are well established in the prior art.
Other applications are concerned with how a magnetic body may be magnetized to a preferred induction level. To establish a preferred induction level, prior art methods generally apply a current with controlled magnitude to a winding (usually with the magnetic body initially in a demagnetized state). The current is understood to produce a magnetic field in the magnetic body that is proportional to current magnitude. Current magnitude (and magnetic field strength) is coordinated with hysteresis curve properties of the magnetic body to produce a preferred induction level. Many different embodiments of this magnetizing method are also well established in the prior art.
Of particular relevance to the present invention is the relationship between voltage induced in a winding and changing magnetic flux. The relationship between induced voltage and changing magnetic flux was originally stated in a general way by Michael Faraday, and is widely known as Faraday""s Law. Faraday""s Law basically states that the voltage induced around a closed path is proportional to the rate of change of magnetic flux within the closed path. It is well established in prior art how Faraday""s Law can be used along with a conductive winding to measure changes of induction level in a magnetic body. However, the prior art does not appear to address how the induction level of a magnetic body can be controlled by controlling the voltage induced in a winding.
The initial motivation for developing the present invention was to provide a way to demagnetize current transformers while the current transformers remain in service. The prior art does not address this problem very well.
Most current monitoring systems for a-c (alternating-current) electric power systems utilize current transformers to provide input currents that are isolated from the electric power system conductors. A primary winding of a current transformer is connected in series with a current-carrying conductor while a secondary winding is magnetically coupled to the primary winding by a suitable magnetic core. A current is induced in the secondary winding that is proportional to the primary current. The secondary current is isolated from the primary current and is smaller than the primary current by the turns ratio of the primary and secondary windings. The primary winding frequently consists of only one turn, which is often just the current-carrying conductor installed through an opening in the middle of the current transformer magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core.
The accuracy of a current transformer is usually related to the coercive force of the magnetic core material (less is better), the cross sectional area of the magnetic core (bigger is better), the effective magnetic length of the magnetic core (shorter is better), any air gap in the magnetic core (less or none is better), and the xe2x80x9csquarenessxe2x80x9d of the magnetic core material hysteresis curve (squarer is usually preferred if not operating near saturation, otherwise characteristics that are not square may be preferred). In the case of high-quality current transformers with very little air gap (usually tape-wound construction, not split-core construction), acceptable accuracy is often achieved as long as operation near saturation is avoided. Split-core current transformer cores generally have hysteresis curves that are less square than standard current transformer cores due to the small air gaps inherent in the design of split-core current transformers.
In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the circuit connected to the secondary winding must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the xe2x80x9cburden.xe2x80x9d The burden generally includes all impedances in the loop through which the secondary current flows, including stray winding impedances, stray impedances of connecting conductors, and the impedances of any devices connected in the loop (such as current-sensing resistors and relay operating coils). In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and is proportional to the burden, in accordance with Ohm""s law. The induced voltage is induced in the secondary winding by a fluctuating induction level in the magnetic core. The fluctuating induction level is associated with a magnetizing current in accordance with well-known electromagnetic principles. The magnetizing current accounts for most of the error in the secondary current. Generally speaking, the accuracy of a current transformer is inversely related to the burden of the secondary circuit. A higher burden causes the secondary current to be a less accurate representation of the primary current.
The accuracy of the secondary current may also be adversely affected by either of the following:
(a) A primary current that is not symmetrical. xe2x80x9cSymmetricalxe2x80x9d is intended to mean that the waveform has positive and negative half-cycles with the same waveform and magnitude. An alternating current that has a d-c (direct-current) component is a common example of a primary current that is not symmetrical. Also, transient a-c fault currents are often not symmetrical. D-c currents are, by definition, not symmetrical.
(b) A burden that is not a linear impedance. Nonlinear burdens are common in applications that derive power from the secondary current.
In either of these cases, the current transformer core may become magnetized. This magnetization may cause significant error in the secondary current. This error may include distortion of the secondary current, including the loss of any d-c component that is present in the primary current.
In accordance with the present invention, a varying voltage is applied to a conductive winding that magnetically interacts with a magnetic body. The varying voltage controls the voltage induced in the winding in such a way that the integral over time of the induced voltage correlates to desired changes of the induction level (magnetization) of the magnetic body. The invention may be used to control the induction level of a magnetic body in several ways:
(a) The induction level of a magnetic body may be caused to transition from a known induction level to a preferred induction level. (A preferred induction level of zero may be chosen to demagnetize a magnetic body).
(b) When the induction level is not known, a preferred induction level may be established by changing the induction level of the magnetic body from an unknown induction level to a known induction level and then to the preferred induction level.
(c) A preferred induction level may be maintained by causing the induced voltage across the winding to have an average value near zero (or by causing the integral of induced voltage to not exceed a predetermined value).
(d) A preferred induction level may be more strictly maintained by causing the induced voltage across the winding to continuously be near zero volts, thereby reducing the amount that the induction level fluctuates.
(e) The induction level may be made to vary with time in a preferred manner, including matching a control signal that is proportional to a reference induction level.
The varying voltage may be generated directly by an active voltage source (or, more generally, a controllable electric energy source) or the varying voltage may be generated indirectly, such as by current transformer secondary current flowing through an adjustable impedance. The key elements are a magnetic body, a conductive winding that magnetically interacts with the magnetic body, and a means of causing the induced voltage to have the appropriate waveform and magnitude.
A conductive winding that is utilized for controlling induction level may be a permanent winding that is also used for other purposes, or it may be a dedicated winding (permanent or temporary) provided solely for the purpose of controlling induction level.
The invention is most readily applied to magnetic bodies that are configured to have a relatively uniform magnetic path, such as the magnetic cores of current transformers. The accuracy of a current transformer may be improved by the invention in three ways:
(a) By demagnetizing a current transformer, inaccuracies associated with core magnetization are removed. A demagnetized current transformer can accurately measure d-c current and a-c current that has a d-c component.
(b) By keeping the integral over time of induced voltage near zero, a current transformer is better able to measure unsymmetrical currents without quickly transitioning to saturation.
(c) By reducing the amount that the induction level in the core fluctuates, inaccuracies associated with magnetizing current may be greatly reduced and the accuracy of a current transformer may be greatly improved.
When utilized with an a-c power system, current transformers may be demagnetized while in service by utilizing an adjustable impedance connected in series with current transformer secondary current. The adjustable impedance causes an induced voltage across the secondary winding of the current transformer. By sequentially adjusting the adjustable impedance, the induced voltage is caused to vary in such a way that the magnetic core of the current transformer is demagnetized.
Current transformers may also be demagnetized using an active voltage source to generate the varying voltage, in which case the varying voltage may also be used to reduce induction level fluctuations required for operation, thereby improving the accuracy of the current transformer.
The invention is based on two principles of electromagnetism:
(1) the relationship of magnetomotive force to flux density as defined by hysteresis curves of magnetic bodies, and
(2) Faraday""s Law applied to magnetic bodies.
The second principle (Faraday""s law applied to magnetic bodies) clarifies how a voltage induced in a winding is directly proportional to a changing induction level. While prior art shows how induced voltage can be used to measure changes in induction level, the present invention shows how induction level can be controlled by controlling induced voltage. The invention primarily utilizes this second principle relationship to control the induction level of a magnetic body.
To establish a preferred induction level in a magnetic body, the first principle may be used during a first phase to change the induction level of a magnetic body from an unknown induction level to a known induction level. Once the induction level is known, the second principle is used during a second phase to change the induction level from the known induction level to a preferred induction level. The second principle may also be used to maintain a preferred induction level, to reduce the amount that the induction level fluctuates, and to cause the induction level to vary with time in a preferred manner.
Since the initial motivation for developing the invention was to provide a way to demagnetize current transformers, much of the description is directed toward current transformer applications and demagnetizing issues. However, it should be clear that the invention may be applied to magnetic bodies in general, and that the invention may be used to magnetize a magnetic body to any preferred induction level, as well as to maintain a preferred induction level, reduce induction level fluctuations, and cause the induction level to vary with time in a preferred manner. It is not the intent of this disclosure to limit the invention to current transformer and demagnetizing applications.
Accordingly, some objects and advantages of the present invention are:
(a) To provide a way to cause the induction level of a magnetic body to vary with time in a preferred manner.
(b) To provide a way to transition from a known induction level to a preferred induction level.
(c) To provide a way to reduce the amount that the induction level of a magnetic body fluctuates.
(d) To provide a way to maintain a preferred induction level in a magnetic body.
(e) To provide a way to magnetize a magnetic body to a preferred induction level, utilizing a primary winding conducting an alternating current and a secondary winding connected to an adjustable impedance.
(f) To provide a way to demagnetize a magnetic body, utilizing a primary winding conducting an alternating current and a secondary winding connected to an adjustable impedance.
(g) To provide a new way to magnetize a magnetic body to a preferred induction level, utilizing a conductive winding and a controllable electric energy source.
(h) To provide a new way to demagnetize a magnetic body, utilizing a conductive winding and a controllable electric energy source.
(i) To provide a way to demagnetizing current transformers while in service, thereby improving the accuracy of current transformers.
(j) To provide a way to improve the accuracy of a current transformer by reducing the amount that the induction level fluctuates during current-sensing operation.
(k) To provide a way to maintain a current transformer in a demagnetized state once the current transformer has been demagnetized.
(l) To provide a way to utilize ordinary current transformers to measure d-c current.
Further objects and advantages will become apparent from a consideration of the drawings and the following detailed description.